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I've had osteoarthritis for years, but now my doctor told me that I have what my rheumatologist called erosive osteoarthritis. What is that?

— Gwen, Alabama

Ordinary osteoarthritis causes the formation of bone spurs (osteophytes), while erosive osteoarthritis causes erosions of the bones and inflammation. Erosive osteoarthritis is a variant of osteoarthritis that occurs in a minority of patients. It affects primarily the middle and end finger joints (called the proximal and distal interphalangeal joints) and the joint at the base of the thumb (first carpometacarpal joint) and causes redness, swelling and pain of the joints.

There are several reports that inflammatory and erosive osteoarthritis may progress to rheumatoid arthritis (RA). We're not sure how or why this happens, but it should be suspected especially if the knuckles (metacarpophalangeal joints), wrists, the feet and other joints become painful and swollen, and there is morning stiffness. It would then be helpful to do a sedimentation rate test (sed rate), a CRP, and a rheumatoid factor test. Neither sed rate nor CRP is elevated in erosive osteoarthritis, but both usually are in rheumatoid arthritis. Rheumatoid factor is positive in 60-80% of adult patients with RA, but not in osteoarthritis. Persons older than 65 may have a low positive rheumatoid factor. Half of patients with RA and negative rheumatoid factor have another antibody positive or anti-CCP.

In summary, if the pain and inflammation start affecting more joints than the middle and end joints of the fingers, the diagnosis of erosive osteoarthritis should be reconsidered and RA should be considered or ruled out. The correct diagnosis is extremely important because RA is a systemic illness and requires very vigorous therapy early on, or else joint damage occurs.